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Question Number 10836 by okhema last updated on 27/Feb/17

$$solvecos2\theta -3cos\theta =1$$$$foro⩽\theta ⩽2\pi$$

Answered by ridwan balatif last updated on 27/Feb/17

$$\cos 2\theta -3\cos \theta =1$$$$({2\cos }^2\theta -1)-3\cos \theta =1$$$${2\cos }^2\theta -3\cos \theta -2=0$$$$(2\cos \theta +1)(\cos \theta -2)=0$$$$\cos \theta =-1/2\vee \cos \theta =2\left(\mathrm{IMPOSSIBLE}\right)$$$$$$$$\cos \theta =-1/2$$$$\theta =\frac{2}{3}\pi ,\frac{4}{3}\pi$$

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